The present invention relates to turbine bucket damper pins and, more particularly, eliminating or reducing bucket Hi-C undercut by incorporating a scallop section in a bucket damper pin.
In a turbine bucket, at a given cross-section, the point at which the gas flow reverses its direction on the convex side of the airfoil is known as the airfoil Hi-C point. Particular interest is generally of the Hi-C point at the root cross-section, known as the root section Hi-C point, since the stress at this location is generally higher than its surrounding locations. With reference to FIG. 1, for buckets with a narrow bucket-to-bucket space due to real estate constraints, the Hi-C may be located in such a way that when a bar-type damper pin slot is machined, there will be inevitable undercut 2 at the Hi-C location immediately below the platform (see the dashed line in FIG. 2). The Hi-C location is generally a highly stressed location, and an undercut 2 will further increase the stress at this location through Kt effect and the reduction of wall thickness. For example, analysis has indicated that, for a particular bucket/damper geometry, the Kt could be as high as 5.0.
It would be desirable to construct the turbine bucket damper pin to avoid the undercut while providing an easily-installed assembly geometry.
In an exemplary embodiment of the invention, a damper pin is provided for a bucket damper slot in a turbine. The damper pin, includes slot insertion ends shaped to fit into the bucket damper slot; and at least a first scallop section formed or machined between the slot insertion ends and shaped to receive a bucket shank pocket radial contour at bucket Hi-C. A second scallop section may also be formed or machined diametrically opposed and anti-symmetrical to the first scallop section between the slot insertion ends.
In another exemplary embodiment of the invention, a method of constructing a damper pin for a bucket damper slot in a turbine includes the steps of (a) forming slot insertion ends shaped to fit into the bucket damper slot; and (b) machining a first scallop section between the slot insertion ends. The first scallop section is shaped to receive a bucket shank pocket radial contour at bucket Hi-C.